A rapid, selective and reproducible high-performance liquid chromatographic method for the analysis in plasma of the calcium channel blocking agent, diltiazem, and one of its metabolites, deacetyldiltiazem is described. The method involves extraction with the methyl tert.-butyl ether of the drugs and the internal standard (verapamil), back-extraction into sulphuric acid and reversed-phase chromatography with UV detection. Over a concentration range of 10-1000 ng/ml the average coefficient of variation for diltiazem was 5.4% and for deacetyldiltiazem was 8.3%.